Why is a low flow channel incorporated into the design?
Based on other diversion channel projects, a low flow channel is necessary to concentrate low flows within the diversion channel to minimize erosion and deposition of materials.
Why is the low flow channel being built with a meandering alignment?
During feasibility, the meandering of the low flow channel was discussed at numerous meetings attended by the sponsors, natural resource agencies, Corps, and AE representatives. This topic was the focus of many discussions and was ultimately recommended in the Final Feasibility Report and Environmental Impact Statement. Initial plans included aqueducts (similar to those on the Maple and Sheyenne Rivers) to carry the Rush and Lower Rush Rivers over the diversion. However, as a cost saving measure, this concept was eliminated from consideration and was replaced with abandoning portions of the Rush and Lower Rush River channels and dropping the Rush and Lower Rush Rivers directly into the diversion. Therefore, the low flow channel in the bottom of the diversion would be replacing the Rush and Lower Rush Rivers and would need to convey sediment and flow from these tributaries. It was also determined that meandering the low flow channel to replace habitat values (even if limited) lost from the Rush and Lower Rush Rivers would be desirable. The team worked collaboratively and agreed that the low flow channel could be designed to meander, such meandering would not be cost prohibitive, and it would be more desirable from engineering and environmental standpoints.
The low flow channel would meander on its own. Why then is it being constructed with meandering instead of allowing it to do this naturally?
A meandering channel will minimize undesired erosion. It will also provide immediate benefits that are achieved by having a meandering channel as opposed to waiting several years for the meanders to occur naturally.
What analysis is being performed with respect to the meandering?
A meander migration-rate study is being conducted.
Would this same analyses be required if the low flow channel was not built with meandering?
Yes. We would need to know how much/far the low flow channel would potentially move regardless of whether the channel was straight or meandering.
How much additional cost is anticipated with constructing the meandering?
As mentioned earlier, it is a cost savings to construct a meandering low flow channel versus aqueduct structures on the Rush and Lower Rush Rivers. Although larger, the Maple River aqueduct structure is estimated to cost $60 million.
Are other existing diversions being looked at for lessons learned?
Yes. The West Fargo, Horace to West Fargo, and Winnipeg diversion channels are being investigated, and information gathered from these observations largely lead to the realization that the low-flow channel needs to be larger than identified in feasibility.
If so, what is being done differently for the FM diversion?
The FM diversion will have a larger low flow channel that meanders.
Why is the low flow channel larger than it was in the Feasibility Study? Is it solely due to the meandering?
No. Incorporating meandering does not change the size of the channel. The low flow channel size was increased to accommodate the anticipated low flows in the bottom of the diversion. The benefits of a properly sized low flow channel will result in less erosion and deposition of materials, which will directly relate to lower Operations and Maintenance costs for the sponsor in the future. A low flow channel that is too small causes unwanted deposition and unpredictable erosion that could threaten the stability of the diversion channel’s side slopes. Because natural channels meander, the design team is working to incorporate this natural tendency into the design.